The present invention relates in general to waveform sampling systems and more particularly to a pseudorandom sampling system wherein a repetitive waveform is sampled at a predetermined time following a triggering event in a waveform and wherein the time interval between the sample and a subsequent triggering event is accurately measured.
A typical waveform sampling system repetitively strobes a sampling gate to sample a waveform at several points and the analog samples obtained are converted into digital data and stored in memory. In order to accurately characterize the shape of a sampled waveform, sample data should convey not only the magnitude of each waveform sample but the relative timing of each sample with respect to a triggering event (such as a zero crossing) in the waveform. Sequential and random sampling systems each provide timing information in a different way. Sequential sampling systems typically sample the waveform at predetermined regular time intervals following a triggering event in the waveform being sampled. The sampled waveform magnitude data is stored in memory in the order that it is acquired and since the sample timing is regular and predetermined, the position of the sample data in the memory is indicative of relative timing. In random sampling systems, sampling strobe signals are not synchronized to triggering events in the waveform and therefore the timing of each sampling strobe is "random" with respect to triggering events and not predetermined. Thus in random sampling systems it is necessary to measure the time interval between each sample and a triggering event in the waveform in order to determine the relative timing of each sample. The measured timing data is stored in memory along with the sampled waveform magnitude data.
Sampling systems are also characterized as to whether they perform real time or equivalent time sampling. In real time sampling systems a single section of a waveform is sampled and the resolution of the sampling, i.e., the maximum time between samples, depends entirely on the sampling frequency. Real time sampling is most suitable for non-repetitive or relatively low frequency periodic waveforms.
The equivalent time sampling method is used to obtain data characterizing a repeating section of a relatively high frequency, repetitive waveform. In equivalent time sampling, the waveform is sampled one or more times during each of several successive "acquisition windows", each acquisition window comprising a time period bounding a different repetition of the particular section of the waveform to be sampled. In sequential equivalent time sampling systems, a repetitive triggering event in the waveform occurring at some known time with respect to each acquisition window initiates sampling during each acquisition window. The initiation of sampling is delayed by differing predetermined times after each triggering event so that sampling occurs at different relative times within each acquisition window. The sample data is then ordered according to the relative sampling time within an acquisition window rather than according to the actual order in which the sample data was acquired. In random equivalent time sampling systems, sampling times and triggering events are not synchronized but the time interval between samples within each acquisition window and a triggering event associated with the window is measured.
The resolution of sequential sampling systems depends on the resolution in control over sample timing delay while the resolution of random sampling systems depends on the resolution in measurement of the time differences between sampling strobes and triggering events. In sequential equivalent time sampling the timing of each sample is predetermined and there is essentially a one hundred percent probability that each sample will be taken within an acquisition window. However in random equivalent time sampling systems, the timing of each sample is not predetermined and many samples may be taken outside the intended acquisition window and must be discarded. The controllability of sample timing within an acquisition window afforded by sequential sampling permits samples to be taken at evenly spaced relative times within the acquisition windows such that a minimum number of samples are required to obtain a given resolution while in random sampling systems samples are not necessarily evenly spaced and more samples must be taken in order to achieve the same degree of resolution. Therefore when the sampling frequency for random and sequential equivalent time sampling is comparable, the random sampling method requires more time to achieve a desired degree of sampling resolution than sequential sampling.
Due to practical limitations of sequential sampling strobe drive circuitry, a triggering event must precede an acquisition window by a certain amount of time. If the triggering event is in the acquisition window, samples cannot be taken before the triggering event and therefore the entire window cannot be sampled. A triggering event occurring during one acquisition window may be utilized to trigger sampling for a subsequent acquisition window. However in such systems waveform "jitter" reduces sampling accuracy. Not all repetitive waveforms are periodic since the time between repetitive waveform sections in "jittery" waveforms may vary and the sample timing within an acquisition window may vary from expectations if the triggering event occurs outside the acquisition window. Consequently, in many equivalent time sequential sampling systems the sampled waveform is delayed following trigger pickoff, before being applied to the sampling gate, so that a triggering event within an acquisition window may be utilized to trigger sampling over the full range of the sample of the acquisition window. However, delay circuits may distort some waveforms to an intolerable degree and must be periodically measured to ensure that the delay time is accurately known.
What is needed is a method and apparatus for sampling a waveform with high resolution and high speed which is not subject to error due to waveform jitter and which does not require the delay of the waveform being sampled.